Safety joints are commonly used with work strings including drilling, fishing, testing, wash-over, tubing or other strings. They allow the disengagement of the lower portion of the work string at a pre-determined location or position. These safety joints are important in situations in which, for example, a work string becomes stuck in a wellbore. Often times, expensive equipment or tools are present at the lower end of the work string, the retrieval of which is necessary. Safety joints are, therefore, placed below expensive equipment on the work string to ensure that equipment is retrieved once the safety joint is disconnected. Safety joints are designed to break out at a lower torque magnitude than all the connections in the work string so that if the work string gets stuck, there is a known location and a known torque magnitude for disengagement.
Typical safety joints are tubular in shape and made up of two parts, an upper member, or pin, and a lower member, or box, that are connected by known means, such as, for example, coarse threads. When the safety joint is assembled, right hand torque or rotation causes the pin to axially move into the box. When a work string becomes stuck in a wellbore, left hand torque is applied to the work string to uncouple the pin from the box allowing the retrieval of the pin and the work string above it. The design of the safety joints allows their reconnection downhole via the application of right hand torque.
To avoid wash-out of the threads and the loss of fluid through the work string, two seals, (for example, O-rings), are usually installed on both sides of the threaded connection. When the safety joint is assembled on the surface, there is no wellbore fluid present and hence no problem during assembly of the safety joint. However, when wellbore fluid is present in the environment of the safety joint, particularly in the box, a volume of fluid gets trapped between the aforementioned two seals. This trapped fluid may pose a problem for reengaging the safety joint downhole. During reengagement, the volume between the two seal may be reduced and the wellbore fluid may be compressed, creating what is referred to as a hydraulic lock. This fluid compression, or hydraulic lock, results in an internal reaction force that reduces the tightening of the connection as torque is applied to the safety joint. This reduction of the tightening could cause an operator to assume that the safety joint is safely made up to its required makeup torque, when it is not made up at all. Further, the break out torque required to disengage the connection may be reduced as well, and consequently the safety joint may accidentally disconnect.
Currently, a number of options exist that aim to solve the problem of hydraulic lock between the two seals. For example, one or both of the seals could be removed to prevent trapping and compressing wellbore fluids altogether when reengaging the safety joint. However, this approach has drawbacks. Removing both seals means that there is no way of preventing washout of the threads if there is pressurized wellbore fluid circulating in the work string. Removing just one of the seals would not result in washout, but the life of the threads would be reduced due to corrosion pitting by the wellbore fluid.
There is a need, therefore, for a safety joint designed in a manner that ensures its safe and proper reengagement in downhole environment.